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1. Field of the Invention
The invention is related to the field of telecommunications, and in particular, to systems and methods that identify a polarization-mode dispersion event.
2. Description of the Prior Art
Polarization-mode dispersion (PMD) is a dynamic pulse broadening phenomena. In a single-mode optical fiber, it is understood that optical pulses propagating down an optical fiber will separate into two orthogonal modes of polarization that travel at different speeds. The relative amplitudes of these two pulses is determined by the state of polarization of the input pulse relative to the fiber""s input principal states of polarization (PSP). The separation into the two orthogonal modes is caused by the non-uniformity of the core diameter. This non-uniformity of the core diameter may be a result of imperfections in manufacturing, ambient temperatures, stress on the optical fiber, and/or movement of the optical fiber.
If the core has a perfectly circular cross-section, then both modes will travel at the same speed over the same distance. Otherwise, one mode will travel slower than the other resulting in a difference in group velocities (an effect called birefringence). Like chromatic dispersion, the difference in velocities between polarization modes is wavelength dependent. For PMD, the difference in velocity is also time dependent. The difference in propagation time, xcex94xcfx84, experienced by the two polarization modes at a given wavelength is referred to as the differential group delay (DGD) with units in picoseconds (ps). It is well known that xcex94xcfx84 obeys a Maxwellian distribution. When the DGD in an optical fiber becomes excessively large, the receiver is unable to distinguish between a zero bit and a one bit, and bit errors occur eventually resulting in a PMD-induced outage.
PMD is a time varying stochastic effect. Identification, measurement, and compensation for PMD is difficult because of the time varying stochastic nature of PMD. A fiber operator may have an outage on their fiber that is unknowingly caused by PMD. Due to the time varying stochastic nature of PMD, the outage may resolve itself Thus, the fiber operator may have trouble tickets that are closed or left unresolved even after extensive activity by maintenance employees. The only way of identifying PMD has been through direct measurement of PMD or other analog characteristics of the optical signal. Some prior systems have identified, measured, and compensated for PMD by measurement of DGD and PSP. One such system is disclosed in a pending United States Patent Application, entitled xe2x80x9cMethod And Apparatus To Compensate For Polarization Mode Dispersion,xe2x80x9d filed on Feb. 8, 2000, Ser. No. 09/500,092. Other prior systems monitor eye openings of the optical signal or perform other analog measurements of the optical signal to identify PMD. Another prior system measures the PMD, measures a bit error rate (BER), and correlates the PMD and BER to isolate errors due to the PMD. This system is disclosed in a pending United States Patent Application, entitled xe2x80x9cCorrelating Polarization Mode Dispersion and Bit Error Rate,xe2x80x9d filed on Apr. 25, 2000, Ser. No. 09/558,448. One problem with these prior systems is the cost of identifying, measuring, and compensating for PMD can be expensive. What is needed is a system to identify PMD that is accurate and cost effective.
The inventions solve the above problems by identifying a polarization-mode dispersion event in an optical fiber by determining error conditions in proximate wavelengths. The embodiments of the invention identifies these PMD events without measurement of PMD, DGD, or any other analog characteristic of the signal. A system includes an optical interface and a PMD identification system. The optical interface receives an optical signal from the optical fiber wherein the optical signal is wavelength division multiplexed with a plurality of wavelengths. The PMD identification system determines whether a first error condition exists in a first one of the wavelengths of the optical signal. The PMD identification system also determines whether a second error condition exists in a second one of the wavelengths of the optical signal wherein the second one of the wavelengths is proximate to the first one of the wavelengths. The PMD identification system also determines whether an acceptable condition exists in a third one of the wavelengths of the optical signal that is not proximate to the first one and the second one of the wavelengths of the optical signal. The PMD identification system then determines the presence of the polarization-mode dispersion event in the optical fiber based on the first error condition, the second error condition, and the acceptable condition.
In some embodiments, the first error condition is an error count in the first one of the wavelengths of the optical signal. In other embodiments, the first error condition is signal degradation in the first one of the wavelengths of the optical signal. In some embodiments, the second error condition is an error count in the second one of the wavelengths of the optical signal. In other embodiments, the second error condition is signal degradation in the second one of the wavelengths of the optical signal. In some embodiments, the acceptable condition is an acceptable error count in the third one of the wavelengths of the optical signal. In some embodiments, the acceptable condition is an acceptable signal condition in the third one of the wavelengths of the optical signal. In one embodiment, the processing system determines the probability of the polarization-mode dispersion event in the optical fiber based on a first differential group delay of a receiver system and a mean differential group delay of the optical fiber to improve the confidence of the identification. In some embodiments, the system includes a forward error correction decoder to decode the optical signal with forward error correction. In some embodiments, the system includes a demultiplexer to demultiplex the optical signal with the plurality of wavelengths.
Because PMD has an effect on proximate wavelengths in an optical signal, the system advantageously identifies PMD events by determining error conditions in proximate wavelengths. A high PMD event is localized within a small band of wavelengths. The width of this band is termed the DGD bandwidth. Thus, a PMD event is identified without actually measuring the instantaneous PMD or monitoring the analog characteristics of the optical signal for PMD. As a result, the overall costs of identifying PMD and maintaining a fiber network are reduced.